Summary Air Labrador Flight 8300, a Beech1900D, serial numberUE266, was en route from Stephenville, Newfoundland, to St.John's, Newfoundland, when the right cabin emergency exit window failed. The flight crew completed a rapid descent, requested and received a clearance to return to Stephenville, and declared an emergency. The aircraft landed at 0900 Newfoundland daylight time, without further incident. There were no injuries. Ce rapport est galement disponible en franais. Other Factual Information The aircraft departed Stephenville for St.John's at 08261, with two crew and nine passengers, including an infant, on board. The captain was the pilot flying (PF) and the first officer was the pilot not flying (PNF). At approximately 38nautical miles (nm) from Stephenville, while the aircraft was climbing through 18500feet above sea level (asl), there was a loud bang, and both crew members experienced severe ear discomfort and the first officer experienced dizziness. The first officer contacted Gander area control center (ACC), requested descent, and advised the controller that the aircraft cabin had depressurized. The crew received a descent clearance and the first officer then donned his oxygen mask and switched the microphone selector switch from the normal to the mask position. At about the same time, a passenger approached the cockpit and informed the crew that a cabin window had broken. The passenger then returned to her seat. The captain did not don his own oxygen mask nor did he deploy the passenger oxygen masks. After donning his oxygen mask, the first officer attempted to communicate with ATC; however, he could not hear his voice (sidetone) on his headset and assumed that he was not transmitting. Gander ACC, however, was receiving the transmission. Consequently, the captain intervened and requested a clearance to return to Stephenville. At an altitude of approximately 13000feet and one and one half minutes after the window failed, the first officer removed his oxygen mask and resumed communications using his normal microphone. The first officer then made a cabin announcement to the passengers using the passenger address (PA) system. The time required for descent to 10000feet was approximately five minutes. After levelling off from the rapid descent and assessing the situation, the crew declared an emergency. The first officer initiated the Cabin Decompression emergency checklist; however, the captain redirected him to the Cabin Door or Cargo Door Unlocked emergency checklist. This latter checklist was the only emergency checklist actioned. The crew were able to determine that the third cabin window on the right side of the aircraft had failed, but were unable to establish if there was more extensive damage. The crew carried out an uneventful landing in Stephenville. The company standard operating procedures (SOPs) and the aircraft emergency checklist require that in the event of a rapid cabin depressurization, the crew is to initiate an emergency descent and don their oxygen masks. After the descent and when the aircraft is stabilized in level flight below 13000feet, the Emergency Descent and Cabin Decompression checklists are to be called for and completed. Completion of the correct checklist ensures that important safety procedures have been followed. Examination of the aircraft showed that most of the right cabin emergency exit window and a piece of interior window trim were missing, with only small window fragments still attached to the window seal. The failed window fragments and the two right side forward cabin windows were removed and sent to the manufacturer for further examination. Two of the fragments contained the area of the plastic side window that protruded beyond the rubber seal and was exposed to the environment in a similar manner as the two forward side windows. All the cracked edges appear to be the results of secondary cracking failures propagating from the original crack failure. Both pieces had surface chip gouges on the exterior surface between the rubber seal and the cracked edge. The chip gouges measured 0.022inches deep by 0.050inches wide and 0.028inches deep by 0.075inches wide. Under magnification, cracking vents were observed protruding internally from the chip gouges. The Beech 1900 Maintenance Manual states that, for pressurized flight, the maximum allowable depth for scratches, gouges, or chips in a window is 0.015inches. Examination of the two forward right side windows revealed many light scratches and small chipped gouges that ranged in depth from 0.009to 0.026inches, with one of the gouges reaching a maximum width of 0.125inches. Cabin windows in the aircraft, except for the forward cabin window on the right side, are constructed of single-ply cast acrylic. The forward cabin window uses multi-ply construction because it is located in the plane of rotation of the right propeller and consequently exposed to ice and other debris being thrown from the propeller. When there is rapid cabin decompression and supplemental oxygen is not used, the crew and passengers are at risk of exposure to the effects of hypoxia. Hypoxia is described as a lack of oxygen in the blood and will occur to some extent at altitudes above 10000feet asl. It is caused by the reduction in atmospheric pressure as altitude increases. The effects of hypoxia are wide-ranging and, even at a relatively low altitude, such as in this occurrence, symptoms such as faulty memory and poor judgement can occur. Supplemental oxygen is made available for flight crew and passengers in pressurized aircraft to ensure the effects of hypoxia are minimized in the event of cabin decompression. A search of the TSB accident and incident database did not reveal any other cabin window failures with the Beech1900 series aircraft. The cockpit voice recorder (CVR) was removed and sent to the TSB Engineering Branch for further analysis. Approximately 32minutes of high-quality data was retrieved.